Abstract

Feathers act as vibrotactile sensors that can detect mechanical stimuli during avian flight and tactile navigation, suggesting that they may also detect stimuli during social displays. In this study, we present the first measurements of the biomechanical properties of the feather crests found on the heads of birds, with an emphasis on those from the Indian peafowl (Pavo cristatus). We show that in peafowl these crest feathers are coupled to filoplumes, small feathers known to function as mechanosensors. We also determined that airborne stimuli with the frequencies used during peafowl courtship and social displays couple efficiently via resonance to the vibrational response of their feather crests. Specifically, vibrational measurements showed that although different types of feathers have a wide range of fundamental resonant frequencies, peafowl crests are driven near-optimally by the shaking frequencies used by peacocks performing train-rattling displays. Peafowl crests were also driven to vibrate near resonance in a playback experiment that mimicked the effect of these mechanical sounds in the acoustic very near-field, reproducing the way peafowl displays are experienced at distances ≤ 1.5m in vivo. When peacock wing-shaking courtship behaviour was simulated in the laboratory, the resulting airflow excited measurable vibrations of crest feathers. These results demonstrate that peafowl crests have mechanical properties that allow them to respond to airborne stimuli at the frequencies typical of this species' social displays. This suggests a new hypothesis that mechanosensory stimuli could complement acoustic and visual perception and/or proprioception of social displays in peafowl and other bird species. We suggest behavioral studies to explore these ideas and their functional implications.

(A) A peahen (foreground) with the plane of her crest oriented towards the displaying peacock (background) as he performs train-rattling vibrations. (B) Both sexes have a crest with an inverted pendulum shape made up of between 20–31 feathers. This photo shows an adult male measured in vivo. (C) A single crest feather showing the pennaceous flag at the distal end. Note that only short, thin barbs are present on the relatively bare rachis (shaft) at the proximal end. (D) A whole crest sample mounted for the laboratory experiments. The two axes of vibrational motions (“in-plane” and “out-of-plane”) are indicated. (E) Mechanosensory filoplumes (circled) are located at the base of the peafowl crest feathers.

Length and width of the whole crest samples as compared to live peafowl crests.

Crests (n = 8 female, n = 7 male) measured in vivo (means shown to the right of each data column) had similar morphology to the dried samples, except that the crests on live birds tended to be wider. Dried sample dimensions were measured to the nearest 0.1 cm. Each crest sample is indicated by a unique symbol-color combination consistent with other figures (see for details).

(A) Vibrational spectrum and Lorentzian fit for peacock crest sample Crest 01. (B-D) Data on the mean crest resonant frequencies, fr, and quality factors, Q. Each dried crest sample (n = 8 female, n = 7 male) is indicated by a unique symbol-color combination, consistent with . (B) The mean resonant frequencies, fr, of the crest are a close match for the range of vibrational frequencies used during peafowl social displays. As an indication of measurement error, the average 95% CI for each mean fr estimate spans 0.072 Hz. The gray shaded area is the range of vibrational frequencies of the train-rattling display, with dotted lines showing the means for displays performed by peacocks (blue) and peahens (green) []. Variation in fr was influenced by the vibrational orientation and was also associated with the sex of the bird, but there was no significant association with the area of pennaceous flags at the top of the crest. The first panel in (B) also shows how a small sample of single crest feathers (n = 3 from male Crest 03, n = 5 from male Crest 05, and n = 3 from female Crest 10) had a similar range of resonant frequencies as the whole crests vibrated in the same out-of-plane orientation. (C) Fundamental frequency for vibrations in the out-of-plane orientation for peafowl crest and non-crest feathers with similar lengths and crest feathers from four non-peafowl species described in and . Means for male and female peafowl crests are both plotted. The y-axis of (C) is aligned with that of (B) for comparison. (D) The mean quality factor, Q, was also influenced by the vibrational orientation, and was associated with the sex of the bird and the area of pennaceous flags. The average 95% CI for each mean Q estimate spanned 0.233. Black horizontal lines in (B) and (D) are grand means.

(A) Time series showing the change in flag position after a peacock crest (Crest 09) was impacted by a moving vortex of air. When peafowl crests were impacted by such air ring vortices, they deflected measurably, oscillating at their resonant frequency with an amplitude that decayed to a few percent of the initial value over the period of the peacock’s wing-shaking display. (B) Mean resonant frequencies (fr) and mean vortex response frequencies (± 95% CI) for three crests in the vortex experiment.

(A) An example waveform and spectrogram of the train-rattling sound used in the playback experiment. The white box in (B) highlights a single rattle note in the train-rattling spectrogram. (B) Vibrational response of a peahen crest (Crest 08) exposed to audio playback in the near-field of the speaker. The FFT spectral power during playback of train-rattling sound (dotted line, plotted on a linear scale on the y-axis) has a peak near the resonant frequency of the crest. The spectral power values recorded during white noise playback (solid line) and when the train-rattling audio was blocked by a foam tile (red dashed line) are also shown.

Vibrational response of a female peahen crest (Crest 13) exposed to airflow from a robot that simulated 5.0 Hz peacock wing-shaking displays at a distance 50 cm from the moving wingtip (see also ). Note that the FFT spectral power (y-axis) is plotted on a linear scale.